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Well, THAT's Not Gonna Fucking Work!

As you may have read, Phil Graham and I are putting together a curriculum for an advanced measurement and array seminar to be taught in early 2012. I sat down this afternoon to try and pound out some measurements of electrical filter behavior: different high and low pass classes and slopes, combining behavior, phase differences, etc. Since I don't have any DSP lying around I borrowed a dbx DriveRack PA+ from a company I was working for last week. My first step was to double check its electrical response with no filters engaged, and what I saw was so bad I double checked my own interface to make sure I wasn't crazy! This is a fresh program loaded with no settings of any kind: XO, EQ, Delay, etc are all disabled.

You'll probably want to click on that image to blow it up and see clearly... The reasonable looking trace is a hard wire loopback test of my Smaart I/O, which is about 30° laggy and 1dB down at 13Hz; totally acceptable for anything I use it for.

The purple trace is the LOW output of the DriveRack PA+ which, as you can see, has totally wonky phase behavior and is polarity inverted! The green trace is the HIGH output of the DSP which has different phase behavior in the high end (with the same delay time!) and isn't polarity inverted, so the two are about 90° out of phase at 16kHz and 180° out of phase over essentially the rest of the unit's bandwidth. FYI, the word "polarity" doesn't even appear in the manual, as far as I can tell there is no way to invert the polarity of an output, purposefully or not... not like I ever do that when I'm building a loudspeaker preset!

So dbx has managed to value engineer a DSP so hard that its behavior is atrocious compared to even the first DSPs from the early 1990s. I can't use this processor to generate plots for my slides, as the phase behavior is just as important as the magnitude to our discussions and this POS warps it by a totally unreal amount. How can a company like dbx let these things out the door? Who is doing their product design?

P.S. This explains the DriveRack PA trace I got back when I was collecting DSP measurements for my article, which was also polarity inverted and also had funny HF behavior. I wrote it off as a cabling error and since I didn't end up using it in the article didn't worry about it. The goddamn Behringer DSP measures WAY better than these things.

Re: Well, THAT's Not Gonna Fucking Work!

I've measured several 260s, and they are fine with respect to what Bennett sees here. It is the only drive rack I have recommended to people.

The 260 still suffers from inadequate parametrics, infuriating UI, lousy software, and pointless features that I wish were allocated to more parametrics/allpass/etc. It may not be a great product, but once you set it up, it does what it is supposed to do.

-Phil

An Aside:
I've had 3 "which DSP" discussions with people in the past couple weeks, and the field is increasingly sparse. For a fixed signal path DSP at a mid market price, the Xilica XP product is the only one I can feel comfortable with, though it also has pretty lousy software. The Lake LM is the only new fixed signal path DSP that is really top kit.

There are plenty of good flexible signal path "drag and drop" dsps that normally are used in the install world. I've all but given up on fixed path products. If A&H produced a loudspeaker-centric iLive rack box with parametrics, allpass, xo, etc. that was controllable from their iPad app, that would be a killer loudspeaker DSP.

This is of course an outgrowth of loudspeaker manufacturers now producing really good black box processors, or in-amplifier processing. I am all for the trend, but it leaves people without black box system controllers increasingly looking about.

Re: Well, THAT's Not Gonna Fucking Work!

This is simply measurement error. There are a whole host of issues which could be causing problems at the low and high end with this type of unbalanced measurement. Notice that in the mid band the phase is fine.

Re: Well, THAT's Not Gonna Fucking Work!

Let me reply to my reply. I guess I just assumed unbalanced. Probably not, but the amount of noise I'm guessing that is present above 2k suggested it. The two phase lines start out about 120deg out of phase. Their slopes seem pretty near equal thus their total "delay" is equal. At the beginning the green high output is descending quicker. There could possibly be some DC blocking stuff in the dbx for the high output? I would measure all three channels simultaneously with a piece of software which doesn't use noise as a stimulus and compare.

On another note, I agree that the Xilica XP/XD products function well for their price. I have measured them extensively with my software, not SMAART, and never seen anything like what your picture shows above.

Re: Well, THAT's Not Gonna Fucking Work!

Originally Posted by Mark DeArman

This is simply measurement error. There are a whole host of issues which could be causing problems at the low and high end with this type of unbalanced measurement. Notice that in the mid band the phase is fine.

Re: Well, THAT's Not Gonna Fucking Work!

Well I just love how everyone is so quick to blame the DSP. There are like a hundred other things which could be going on. I sure don't have any bias "toward" driverack at all. I think they are pretty junky after my experiences with noisy Driverack 480s.

Re: Well, THAT's Not Gonna Fucking Work!

Bennett-what you are missing is the fact that they are doing you a FAVOR. Or so they think.

I guess they figure that the average user of that quality DSP would use the same crossover points and would not know to invert the polarity on adjacent passbands.

But is not always needed in real life.

I know of a number of other DSP "tricks" that some DSP manufacturers put in the device (and do not tell your about-and you cannot change) in order to 'help you out".

That may be a help of people who don't knowhow it all works, but really gets in the way of others.

And you should not automatically "assume" that the layouts are "normal".

For example-in the Lake processor that is inside the Lab Gruppen amps, the limiters are PRE crossover-eq-level etc.

So they can be very tricky to set-you have to really look at the impedance of the loudspeaker-along with the eq curve to determine how much signal is being sent to the loudspeaker in regards to where a limiter should be set.

Danley Sound Labs

PHYSICS-NOT FADS

Any complicated question can be easily answered by a Simple-Easy to understand- WRONG answer!

Re: Well, THAT's Not Gonna Fucking Work!

While I have a bunch of DR260's on monitor duty, I would not use them for anything more except in an emergency. I always avoid the DRPA like the plaque. It's a known piece of junk. With all the changes at Harman and my personal difficulties getting Crown service, I do what I can to steer away from Harmon products.

Re: Well, THAT's Not Gonna Fucking Work!

Originally Posted by mark dearman

well i just love how everyone is so quick to blame the dsp. There are like a hundred other things which could be going on. I sure don't have any bias "toward" driverack at all. I think they are pretty junky after my experiences with noisy driverack 480s.

Re: Well, THAT's Not Gonna Fucking Work!

Originally Posted by Ivan Beaver

For example-in the Lake processor that is inside the Lab Gruppen amps, the limiters are PRE crossover-eq-level etc.

So they can be very tricky to set-you have to really look at the impedance of the loudspeaker-along with the eq curve to determine how much signal is being sent to the loudspeaker in regards to where a limiter should be set.

Hi Ivan,
I know that the module limiters are pre-everything, but the voltage limiters on the outputs are post everything. That's where you can really dial things in.

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